Embodiments of the invention relate generally to a radio frequency (RF) micro-electromechanical systems (MEMS) package, and more particularly to a RF MEMS package having an inverted or flipped ground plane design that forms a RF transmission line for a MEMS device that does not include its own integrated ground path.
RF MEMS devices are a technology that in its most general form can be defined as miniature devices that use an electrically actuated mechanical movement to achieve an open circuit or a closed circuit in a RF transmission line. When the RF MEMS device is in an on-position, the RF transmission line is “closed” and the RF MEMS device can be used to transmit a high-frequency RF signal.
One known technique for fabricating a RF transmission line is through the use of a microstrip transmission line 10, as shown in FIG. 1. Microstrip transmission line 10 includes a dielectric substrate 12 having a conductive signal line 14 positioned on a top surface 16 of the dielectric substrate 12 and a ground plane 18 positioned on a bottom surface 20 of the dielectric substrate 12. Conductive signal line 14 and ground plane 18 interact with each other to create an electromagnetic wave that travels through dielectric substrate 12 to create a RF signal. The width of the microstrip transmission line 10, the thickness of the dielectric substrate 12, and the relative dielectric constant of the material of the dielectric substrate 12 determine the characteristic impedance of the microstrip transmission line 10. A RF MEMS device may be included in the conductive signal line 14 so as to control whether the RF transmission line is “open” or “closed.”
Microstrip transmission lines such as that illustrated in FIG. 1, are often used in conjunction with a conductive-backed coplanar waveguide transmission lines in high-frequency circuit designs. That is, the RF transmission line may transition from being in the form of a microstrip transmission line to being in the form of a conductive-baked coplanar waveguide transmission line.
FIG. 2 shows a conductive-backed coplanar waveguide transmission line 22 as known in the art. Coplanar waveguide transmission line 22 includes conductive signal line 14 positioned on the top surface 16 of the dielectric substrate 12. When transitioning between a microstrip transmission line 10 to a coplanar waveguide transmission line 22 on the same dielectric substrate 12, signal line 14 continues across the top surface 16 of the dielectric substrate 12 through the transition. Coplanar waveguide transmission line 22 further includes and a pair of ground lines 28, 30 positioned on the top surface 16 of the dielectric substrate 12 and on either side of the conductive signal line 14. As such, the signal line 14 and ground lines 28, 30 are positioned on the same side, and, therefore, are coplanar. In addition, ground plane 18 is positioned on the bottom surface 20 of the dielectric substrate 12, and similar to signal line 14 continues across the bottom surface 20 of dielectric substrate 12 through the transition. Coplanar waveguide transmission line 22 further includes a plurality of grounding vias 36 formed through the thickness of the dielectric substrate 12 so as to electrically connect ground lines 28, 30 and ground plane 18. A RF signal is created by an electromagnetic wave between signal line 14 and ground plane 18 and also between ground lines 28, 30 and signal line 14.
As discussed above, RF transmission lines used in a high-frequency circuit design include a ground plane that is integrated onto the bottom surface of the mounting substrate of the MEMS device. The integrated ground plane interacts with the signal line and creates an electromagnetic wave and RF signal. However, because not all MEMS packages are manufactured with an integrated ground plane, existing MEMS packages must be modified prior to being integrated into devices for RF transmission. Such a modification includes positioning a ground plane on the surface of the mounting substrate opposite the MEMS device and forming vias through the substrate to electrically connect the ground plane to the ground lines on the top surface of the substrate. However, the creation of vias in substrates made from materials such as quartz and silicon can be difficult and time consuming.
Therefore, it would be desirable to integrate a MEMS device that does not include an integrated ground plane into a RF MEMS package without structural modifications to the MEMS device or its associated mounting substrate.